Method for producing multilayer tailored fiber placement (tfp) preforms using meltable fixing fibers

ABSTRACT

A method is described whereby multilayer tailored fiber placement (TFP) preforms of any desired thickness may be easily produced without fixing threads or intermediate layers interfering. To this end, the invention provides that reinforcing fibers ( 1 ) are sewn onto a substrate ( 2 ) using a chemically or thermally meltable fixing thread ( 3 ) thus resulting in the formation of a reinforcing fiber structure ( 5   a ). The fixing thread ( 3 ) firstly serves to fix the reinforcing fibers ( 1 ) on the substrate ( 2 ) and is subsequently melted so that the fixing thread ( 3 ) disintegrates while pre-fixing the reinforcing fibers ( 1 ) and does not influence the mechanical properties of the reinforcing fiber structure ( 5   a ).

[0001] The present invention relates to a method for producingmultilayer tailored fiber placement (TFP) preforms using meltable fixingthreads, in particular for the production of TFP preforms of any desiredthickness and without interfering intermediate layers.

[0002] A known method of producing tailored fiber placement (TFP)structures is to sew reinforcing fibers onto a substrate. However, thethickness of an individual TFP structure is limited to approximately 5mm because sewing additional reinforcing fibers onto the structurealready formed damages the fibers previously sewn on. In addition, thesewing threads used previously to fix the reinforcing fibers, as well asthe sewing substrate and the under-threads accumulated on the bottom ofthe substrate material remain in the TFP structure. When multilayer TFPstructures made up of more than two individual TFP structures areproduced, at least one of these layers containing high amounts of fixingyarn remains within the structure in addition to the fixing threads.

[0003] Previously, it was possible to form a preform without anintermediate layer from a maximum of two TFP structures, the twoindividual TFP structures being joined together in such a way that thesubstrate and under-thread accumulation are always located on theoutside of the formed structure. At the same time, this means that sucha preform formed from two individual TFP structures has a maximumthickness of approximately 10 mm. However, in this case also, the sewingthreads are still contained in the fiber composite part afterimpregnation and curing and represent imperfections in the material. Theconstrictions and displacements of the reinforcing fibers caused by thefixing threads and the usually poor connection of the fixing threads tothe matrix material have an adverse effect on the mechanical propertiesof the material.

[0004] A moldable, multiaxial reinforcing structure is known fromEuropean Patent Application 0 567 845 A1, it being possible to positionthe reinforcing fibers in any direction appropriate to the stressconditions using embroidery technology. The glass transition temperature(or softening point) of the embroidery yarns is in this case above thesoftening point of the composite material in order to ensure a securefixing of the reinforcing fibers within the composite until the finalmolding.

[0005] In addition, German Patent Application 196 28 388 A1 describes aforce flux-appropriate, multiaxial, multilayer fiber preform havingZ-axis reinforcement at least in some areas and a method ofmanufacturing same. Z-axis reinforcing fibers are incorporated byembroidery at least in some areas to accommodate the force flux in theZ-axis.

[0006] The object of the present invention is thus to devise a methodmaking it possible to produce multilayer TFP preforms of any thicknessand without interference by fixing threads or intermediate layers in asimple manner.

[0007] This object is achieved by a method having the features ofClaim 1. Additional advantageous embodiments of the present inventionare found in the dependent claims.

[0008] The central idea is that reinforcing fibers are sewn onto asubstrate using a chemically or thermally meltable fixing thread so thata reinforcing fiber structure is produced, the fixing threads firstbeing used to fix the reinforcing fibers to the substrate and then beingmelted so that the fixing threads disintegrate, pre-fixing thereinforcing fibers, without influencing the mechanical properties of thereinforcing fiber structure.

[0009] The use of such a fixing thread has the advantage that, forexample, the constrictions and displacements in the fibers caused bysewing are released again after the melting so that the fixing threadhas no interfering influence on the mechanical properties of thestructure. The release of the constrictions and displacements alsoresults in lower fiber waviness and thus better fiber utilization.Moreover, with partial or complete disintegration of the fiber, nointerface is present from which premature cracks could originate.

[0010] Additional advantages of the fixing thread used for sewing arisefrom the adhesive effect of the melted fixing fiber. This pre-fixes thereinforcing fiber structure so that the reinforcing fiber structurereceives adequate stability for further processing. The advantagesassociated with this are evident primarily in connection with the secondand third embodiments.

[0011] According to a first embodiment, a multilayer TFP preform isproduced in such a way that at least two reinforcing fiber structuressewn together by meltable fixing threads are stacked to form amultilayer stack structure. Subsequently the fixing thread is melted sothat it disintegrates completely in the stack structure. After the stackstructure is impregnated and cured, a multilayer TFP preform withoutfixing threads is obtained.

[0012] Advantageously, the fixing thread may be disintegrated indifferent ways, depending on the type of thread used. For example, thefixing thread may be melted by a chemical reaction with the resin usedfor impregnating or curing the multilayer TFP preform. In so doing, thefixing thread is disintegrated chemically in the matrix used forimpregnating so that it is no longer present in the finished, cured TFPpreform and is thus not able to have an interfering influence on themechanical properties due to constrictions or displacements. Thisprocedure is economical and is advantageous in particular when a TFPpreform is to be produced in as few steps as possible because thedisintegration takes place automatically during the impregnation andcuring process.

[0013] This manner of melting is used not only in the production ofmultilayer TFP structures having an intermediate layer of a sewnsubstrate, but also in the production of TFP preforms withoutintermediate layers, which are made up of either one or two individualreinforcing fiber structures joined like a sandwich, it being possibleto peel off their outward facing substrates after the impregnating orcuring step.

[0014] As an alternative, the fixing thread may be melted by an externalheat source. In doing so, the fixing thread is heated to a temperatureabove its melting point. The melting of the fixing thread by an externalheat source is advantageous in particular when it is desired todisintegrate the fixing thread even before the impregnation and curingprocess. As a result, it is possible to make corrections on an alreadystacked structure before the final impregnation and curing process.Consequently, this type of heating or melting makes more flexiblehandling possible.

[0015] In addition to the use of external heat sources, it is alsopossible to utilize the heat during the impregnation and curing process.This means that the fixing thread melts due to the heat arising duringthe impregnation or curing process. In this case also, the melting andimpregnating/curing take place in one step.

[0016] According to a second embodiment, the fixing thread is melted byapplying external heat immediately after the reinforcing fibers are sewnonto the substrate. A plurality of such reinforcing fiber structures isthen stacked to form a multilayer stack structure, which is thenimpregnated and cured. Advantageously, in addition to disintegrating thefixing thread, the reinforcing fibers are also pre-fixed using theadhesive action of the thermally melted fixing thread, which imparts anadequate stability to the reinforcing fiber structure without theformation of snarls. In other words, it is not only possible to melt thefixing thread after stacking several reinforcing structures but insteadeven before stacking.

[0017] In this manner, it is possible to produce not only thick,multilayer TFP preforms having intermediate layers but also TFP preformswithout intermediate layers, made up of two reinforcing fiber structuresjoined like a sandwich, the substrates in each case facing outwardsand—similar to a two-layer TFP preform according to the firstembodiment—and being removable by simply tearing them off after thefixing thread is disintegrated.

[0018] According to a third embodiment, a multilayer preform of anythickness without an interfering intermediate layer made of substrateand fixing threads is produced, as in the previous cases, by firstsewing reinforcing fibers onto a substrate using a thermally meltablefixing thread so that a reinforcing fiber structure is produced. Next,the fixing thread is heated using an external heat source to atemperature above its melting point. The fixing thread melts, and thereinforcing fibers are pre-fixed due to the adhesive action of themolten fixing thread. This advantageously makes it possible for thesubstrate to be then separated from the thus formed reinforcing fiberstructure and the pre-fixed reinforcing fibers have an adequatestability so that they can be stacked to form a multilayer structure.The structure formed thus has the advantage that it contains nointerfering intermediate layers made up of the substrate and accumulatedfixing threads.

[0019] The fixing thread used for sewing is advantageously athermoplastic thread or a hot melt yarn. An example of a hot melt yarnthat disintegrates when heated above its melting point is a copolyamidemultifilament hot melt yarn (Grilon K85 110 dtex). These threads oryarns ensure a disintegration and an adequate pre-fixing of thereinforcing fiber structure due to the adhesive action. Furthermore, theuse of thermoplastic threads may be advantageous for the mechanicalproperties since brittle resins are normally modified to beimpact-resistant by the addition of thermoplastics.

[0020] Another advantage is that the reinforcing fibers may be situatedon the substrate in force flux orientation so that depending on thedesired application, the reinforcing fibers are aligned on the substratein the desired principal direction of stress. Using the idea of thepresent invention, it is thus also possible to transfer this forceflux-appropriate fiber deposition to multilayer TFP preforms of anythickness. In addition, it is possible to design a quasiisotropicstructure in an analogous manner.

[0021] The invention will be described in greater detail below on thebasis of the appended drawing in which:

[0022]FIG. 1 shows a depiction to explain the application of reinforcingfibers on a substrate material;

[0023]FIG. 2 shows a sectional view of the system shown in FIG. 2;

[0024]FIGS. 3a-3 c show multilayer TFP preforms having intermediatelayers made up of a substrate and an under-thread accumulation accordingto the related art;

[0025]FIG. 4 shows a schematic representation of the multilayerstructure according to the present invention of a TFP preform of anythickness without intermediate layers and

[0026]FIGS. 5, 6 show sectional views of a sample taken from a TFPpreform produced according to the present invention, shown on differentscales.

[0027]FIG. 1 shows a schematic representation to explain the applicationof a reinforcing fiber 1 on a substrate material 2. Reinforcing fiber 1is at first placed on substrate 2 in the desired principal direction ofstress and then sewn onto substrate 2 using a fixing thread 3. Thesewing is done using known methods, reinforcing fibers 1 being attachedto sewing substrate 2 using a zigzag stitch. Substrate 2 may be atear-off fabric or a glass fiber fabric having a surface density ≦100g/m² or even another suitable material. Reinforcing fibers 1 are usuallyrovings of glass and carbon filaments.

[0028]FIG. 1 shows only a first layer of reinforcing fibers, thereinforcing fibers being arranged unidirectionally. It is possible tosew an additional reinforcing fiber layer onto the structure alreadyformed in the same manner. The orientation of the reinforcing fibers mayeither correspond with that of the first layer or, depending on thedesired application, it may also assume a different angle of orientationwith respect to the first reinforcing fiber layer. This means that,depending on the application, the reinforcing fibers may be sewn onunidirectionally in the principal direction of stress or consistent withthe desired force flux orientation.

[0029] The superposition of reinforcing fibers is, however, limitedbecause when new reinforcing fiber layers are sewn on, the needle (notshown) guiding the fixing thread passes through the already sewn-onreinforcing fibers, which may result in damage, constriction ordisplacement of the already sewn fibers. In addition, with increasingthickness of the structure, there is the danger that the needle willdamage the fibers in the lower layers when penetrating the alreadysewn-on fibers. For this reason, the thickness of the layer structuremade up of reinforcing fibers is limited to approximately 5 mm.

[0030]FIG. 2 shows a schematic cross-section of a structure made up ofseveral superimposed reinforcing fibers. Three reinforcing fiber layers1 a, 1 b, 1 c, which are sewn onto substrate 2, are indicated. Thefixing thread is indicated by a dashed line 4 in FIG. 2 only as anunder-thread accumulation on the bottom of substrate material 2. Thisstructure, made up of reinforcing fiber layers 1 a, 1 b, 1 c, substrate2 and under-thread accumulation 4, is identified in the following asreinforcing fiber structure 5 a.

[0031] If a TFP preform is to be produced from such a reinforcing fiberstructure 5 a as shown schematically in FIG. 2, this is done accordingto the related art in such a way that either several individualreinforcing fiber structures 5 a are superimposed randomly, as suggestedschematically in FIGS. 3a and 3 b, or two individual reinforcing fiberstructures 5 a are joined like a sandwich, substrates 2 includingaccumulated under-threads 4 each facing outwards (see FIG. 3c).Subsequently stack structures 6 a formed in this way are impregnated andcured using a known method. However, this means that the known TFPpreform illustrated in FIG. 3a and 3 b has a layer of substrate material2 and accumulated under-threads 4 located between the individualreinforcing fiber layers. Moreover, a preform without an intermediatelayer according to the related art shown in FIG. 3c is limited to amaximum thickness of approximately 10 mm. In addition, the fixingthreads (not shown) are contained in the known structures, which due tothe constrictions and displacements resulting from the sewing process,may have an adverse influence on the mechanical properties of thestructure.

[0032] Instead of the customary yarn materials for fixing reinforcingfibers 1 on substrate 2, a meltable fixing thread is used according tothe present invention. The fixing fiber is distinguished in particularby the fact that it either reacts chemically with the substance used forimpregnation (typically a resin system) or is completely disintegratedby the application of external heat. This results in adequate fixationon the substrate when sewing on, since in this case the thread is usedfor the customary fixation and also the fixing thread disintegrates sothat constrictions caused by sewing are released again and the fixingthread exerts no interfering influence on the mechanical properties ofthe finished, i.e., impregnated and cured TFP preform.

[0033] In addition to using the chemically disintegrating thread toproduce a single-layer TFP preform, it is also used to produce a TFPpreform made up of two or more reinforcing fiber structures (see FIGS.3a-3 c). Reinforcing fibers 1 are sewn onto a substrate 2 using thefixing thread so that reinforcing fiber structure 5a shown in FIG. 2 isproduced. In the next step, for example, it is possible to join tworeinforcing fiber structures 5 a according to the system in FIG. 3c likea sandwich in such a way that each of substrates 2 faces outward. Thetwo-layer sandwich structure formed in this way is then impregnated andcured using a resin system. Because of the chemically disintegratedfixing thread, the finished TFP preform contains no fixing threads 3 orunder-threads 4 after the impregnation and curing process, so that it ispossible to remove substrates 2 by simple tearing off or peeling.

[0034] Likewise, for applications in which intermediate layers made upof substrates do not interfere, a plurality of reinforcing fiberstructures 5 a may be stacked in any manner to a desired thickness, asshown by way of example in FIGS. 3a and 3 b. As in the case describedabove, the structure formed is then impregnated and cured while thefixing thread is chemically disintegrated.

[0035] Instead of a chemically meltable fixing thread, a thermallymeltable fixing thread having a low melting point may also be used tosew on the reinforcing fibers. A fixing thread of this type is, forexample, the hot melt yarn Grilon K85 110 dtx from EMS Chemie, which hasa melting point of approximately 85° C. It might also be noted that themelting point of the aforementioned fixing yarn meltable by chemicalreaction is significantly higher. Of course, the TFP preforms describedabove may also be produced using a thermally meltable fixing thread. Inthis case, the fixing thread is melted by heat, which is described ingreater detail below.

[0036] Moreover, intermediate substrate layers are undesirable orinterfering for many applications so that they must be removed in asuitable manner. To this end, reinforcing fibers 1 are sewn ontosubstrate 2 using a thermally meltable fixing thread having a lowmelting point. Using a meltable fixing thread of this type in turnensures an adequate fixation of fibers 1 on substrate 2 and thedisintegration of thread 3 by the application of heat and itsadhesability result in an adequate stability or strength of thereinforcing fiber layers, so that substrate 2 may be easily torn offfrom reinforcing fibers 1 without in any way adversely affecting thelayer structure of the reinforcing fibers. The result is a reinforcingfiber structure without a substrate as denoted by reference numeral 5 bin FIG. 4. In addition, a stack structure 6 b is shown in FIG. 4, whichis made up of a plurality of reinforcing fiber structures withoutsubstrate 5 b. After impregnation and curing, a multilayer TFP preformof any thickness and without intermediate layers is thus obtained in asimple manner.

[0037] Depending on the application or as a function of the desiredstructure, the heat required to disintegrate or melt the fixing threadmay be applied in various ways. For example, the fixing thread may bemelted by applying heat during the impregnating or curing process.Reinforcing fibers 1 sewn onto substrate 2 using fixing thread 3 areimpregnated using, for example, a heat-curing resin system (e.g., HexcelRTM6, infiltration temperature approximately 120° C.; curing temperatureapproximately 160° to 180° C.) and subsequently cured. Due to the hightemperature during the impregnating or curing phase, the fixing threadmelts and disintegrates in the resin. Since the fixing threads disappearin this way, the substrate may be easily separated from the laminateafter curing by tearing off. In addition the constrictions anddisplacements in the fiber caused by the sewing process are releasedagain. This therefore results in a laminate that is made up exclusivelyof reinforcing fibers and matrix (resin with disintegrated threads).

[0038] It is of particular advantage to utilize the application of heatfor the melting of the fixing thread during the impregnating or curingprocess if a TFP preform is to be formed that is made up of only one ora maximum of two of the TFP structures shown in FIG. 2. In this case, anadditional, separate heating step may be eliminated since thedisintegration of the fixing thread and the impregnation or curing aretied together, resulting in savings.

[0039] For the production of multilayer TFP preforms of any desiredthickness without intermediate layers, the heating is performed in aseparate step preceding the impregnation or curing of the reinforcingfibers. To this end, the structure shown in FIG. 2 is heated underpressure to a temperature above the melting point of the fixing fiber.This may be done, for example, using a heatable press, an externalheating device or other suitable means to heat the structure. Inaddition or instead, controlled amounts of heat may also be applied toreinforcing fiber structure 5 a from the back, so that the sewing threadon the bottom begins to melt and substrate 2 is released. The fixingthread is thus fused and after cooling fixes the reinforcing fibers byits adhesive action and no longer due to the snarling of threadsproduced in the sewing process. At the same time, the melting of thefixing fibers causes the constrictions and displacements of thereinforcing fibers produced by the sewing process to be released again,which results in lower fiber waviness and better fiber utilization. Thetear-off fabric may now be easily detached from this pre-fixed fiberstructure without the reinforcing fibers changing their position. Inthis way, a pre-fixed structure of reinforcing fibers 5 b is created,which contains no fixing fibers and overall has adequate strength andstability for further processing into TFP preforms (see FIG. 4).

[0040] In the next processing step, reinforcing fibers 5 b produced inthis way are stacked on one another and then impregnated and cured usinga known method to form the finished TFP component. A laminate isproduced, which is exclusively made up of reinforcing fibers and has noconstrictions or displacements of the reinforcing fibers due to the sewnfixing thread. A stacked layer structure of this type made up ofreinforcing fibers without intermediate layers is schematically shown inFIG. 4.

EXAMPLES

[0041] Unidirectional TFP preforms including different yarns for fixingcarbon fiber rovings (Tenax HTS 5331-24K) were produced on the substratematerial (glass fiber screen fabric 80 g/m²) are produced on a CNCsewing system. The yarns used as sewing threads were a meltablecopolyamide multifilament hot melt yarn (Grilon K85 110 dtex), whichtypically has a melting point of 85° C., and a polyamide monofilamentyarn (Transfil 56 dtex), a polyamide multifilament twisted yarn (Serafil100 dtex), and a polyester multifilament yarn, which is customarily usedfor the production of multiaxial fabrics (textured PSE 76 dtex). Itshould be noted that, in contrast to the yarns mentioned first, theyarns mentioned last do not melt at the temperatures occurring infurther processing. As an additional variant, the reinforcing fiberswere sewn onto the tear-off fabric “Super Release Blue” using Grilon K85110 dtex. The carbon fiber rovings were placed parallel to each otherspaced at 3.375 mm and fixed to the substrate using a zigzag stitch witha 4 mm overstitch and a stitch width of 2 mm, a total of 4unidirectional layers one on top of the other. Two of these reinforcingfiber structures or TFP semifinished products were then superimposedoutward with the substrate material and impregnated and cured withHexcel RTM6 using the membrane-supported RI method. Samples werecollected from the laminate plates in the direction of the fibers andsubjected to stress until failure in the tensile test and compressiontest. FIG. 5 shows a section from such a sample in which the reinforcingfibers were sewn onto the tear-off fabric Tenax HTS 5331-24K using thefixing yarn Grilon K85.

[0042] It is remarkable that the carbon fiber content by volume actuallyreached in the laminate plates varied strongly despite identicalproduction conditions—all laminates were produced together in oneautoclave cycle using the same quantity of resin in each case. Thelowest values were reached with Transfil and Serafil (approximately50-52%). Significantly higher values of approximately 58-60% werereached with the textured polyester yarn. The cause for this may be theessentially higher elasticity and flexibility due to the texturing(crimping), which results in a more uniform fiber distribution orreduction in areas high in resin. A further increase in the fibercontent by volume (approximately 62%) is reached with the hot melt yarn,which releases the carbon fiber rovings at the time of infiltration andactually passes over into the matrix. Removing the substrate materialincreases the value to approximately 65 to 67% in the variant shown inFIG. 5 or 6. The comparison of the tensile strengths of the differentsamples shows that a significant increase is attained by using Grilon.This resulted in a 10 to 30% improvement compared to the three standardyarns. The compressive strengths of the Grilon samples are average; thebest value is obtained with Serafil. In contrast, the Grilon sampleshave a rather below average stiffness. In summary, compared to the mostfavorable value using standard yarns, the tensile strength is increasedby 30%, the compressive strength by 23%, the tensile modulus by 8% andthe compression modulus by 26%.

What is claimed is:
 1. A method for producing multilayer TFP preforms,in particular for the production of multilayer TFP preforms of anydesired thickness and without interfering intermediate layers, whereinreinforcing fibers (1) are sewn onto a substrate (2) using a chemicallyor thermally meltable fixing thread (3), so that a reinforcing fiberstructure (5 a) is produced, the fixing threads (3) first being used tofix the reinforcing fibers (1) to the substrate (2) and then beingmelted so that the fixing thread (3) dissolves while the reinforcingfibers (1) are pre-fixed and does not affect the mechanical propertiesof the reinforcing fiber structure (5 a).
 2. The method for producingmultilayer TFP preforms as recited in claim 1, characterized by thesteps: stacking of at least two reinforcing fiber structures (5 a) toform a multilayer stack structure (6 a); melting the fixing thread (3);impregnating and curing the stack structure (6 a).
 3. The method forproducing multilayer TFP preforms as recited in claim 2, wherein themelting of the fixing thread (3) takes place during the impregnation andcuring process of the stack structure (6 a) by chemical reaction withthe material to be impregnated.
 4. The method for producing multilayerTFP preforms as recited in claim 2, wherein the melting of the fixingthread (3) takes place through the application of external heat, thefixing yarn (3) being heated to a temperature above its melting point.5. The method as recited in claim 4, wherein the application of heat isutilized during the impregnation and curing process to melt the fixingthread (3).
 6. The method for producing multilayer TFP preforms asrecited in claim 1, characterized by the steps: melting the fixingthread (3) by the application of external heat after the reinforcingfibers (1) are sewn onto the substrate (2) so that the reinforcingfibers (1) are pre-fixed by the adhesive action of the molten fixingfiber (3) and the reinforcing fiber structure (5 a) has an adequatestability; stacking a plurality of reinforcing fiber structures (5 a) toform a stack structure (6 a); impregnating and curing the stackstructure (6 a).
 7. The method for producing multilayer TFP preforms asrecited in one of claims 2 through 6, wherein outward facing substrates(2) of the stack structure (6 a) formed are removed by tearing off orpeeling.
 8. The method for producing multilayer TFP preforms withoutintermediate layers as recited in claim 1, characterized by the steps:melting the fixing thread (3) by the application of external heat afterthe reinforcing fibers (1) are sewn onto the substrate (2) so that thereinforcing fibers (1) are pre-fixed by the adhesive action of themolten fixing fiber (3) and the reinforcing fiber structure (5 a) has anadequate stability; tearing off or peeling the substrate (2) from thereinforcing fiber structure (5 a), resulting in a reinforcing fiberstructure without a substrate (5 b); stacking a plurality of reinforcingfiber structures without a substrate (5 b) to form a stack structure (6b); impregnating and curing the stack structure (6 b).
 9. The method asrecited in one of the preceding claims, wherein the chemically orthermally meltable fixing thread (3) is a thermoplastic thread or a hotmelt yarn.
 10. The method as recited in claim 9, wherein the thermallymeltable fixing thread (3) is a copolyamide multifilament hot melt yarn.11. The method as recited in one of the preceding claims, wherein thereinforcing fibers (1) are force-flux oriented or situated in aquasiisotropic manner on the substrate (2).